Many public and industrial environments utilize air purification systems to remove contaminants from environmental air. As is known in the art, air purification systems are typically closed-loop systems that incorporate filter(s) to collect and/or absorb a variety of contaminants prior to cycling the air back into an environment. Over time, filters lose their efficacy and must be replaced. For example, filter beds known as sorbent filters are canisters filled with granular sorbents such as activated carbon, zeolites, and silica selected to absorb one or more contaminants of interest.
Once a sorbent filter is saturated with contaminant(s), the sorbent media must be replaced. Colorimetric indicators are used to signal when a sorbent media is saturated. Typically, a colorimetric indicator is placed in a fluid-flow region at or just downstream of a sorbent filter exit/exhaust. Such indicators change color to indicate the presence of a contaminant thereby signaling a sorbent media's loss of efficacy. However, in the presence of high levels of a contaminant and/or interfering substances, conventional colorimetric indicators can be prone to discoloration or bleaching thereby lending themselves susceptible to a false negative interpretation. An interfering substance can be a substance in the system other than the target contaminant. The interfering substance can react or interact with the target contaminant or with the indicator dye to cause discoloration. An interfering substance can also be the target contaminant itself when the target contaminant is present in excess or at high concentration. The above-mentioned false negative result can occur when a colorimetric indicator's color is affected by an interfering substance to the point that an observer can mistakenly interpret the colorimetric indicator as being indicative of a filter's sorbent media that is still effective.